should the size of polygons of a 3d model be the same?
You don't have to care about keeping the same size for all the quads, a topology doesn't need to be homogeneous, only bring edge loops when it's useful, delete all the ones that don't bring anything to the shape, you'll save calculation and your object will be easier to work.
For example look at this topology: You have small quads around the lips, they are bigger around the forehead, and even bigger around the shoulders, because it's exactly what's necessary for these parts, no more, no less.